Nafi' said that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: No marriage is lawful for a man except the marriage of Islam, in which he gives her a dowry, and husband and wife inherit from each other; it is not settled for a fixed term that she is his wife, such that if one of them dies they do not inherit from each other.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14175